1. Field of the Invention
The present invention concerns a field-effect semiconducting device, the gain of which can be controlled by a DC voltage. The control of the gain is achieved by separating the drain metallization into two parts which are separated from each other by a secondary gate metallization, constituting a variable resistor which couples the two parts of the drain variably. The transistor according to the invention thus has two channels and two gates, but the two gates are not in the same channel.
2. Description of the Prior Art
Two-gate or multiple-gate field-effect transistors are well-known but, as a rule, the gates are parallel to one another and interposed in one and the same channel between a source and a drain. Furthermore, the fact of placing several gates that exert their effect on a single channel is used in logic circuits to make AND gates for example. When prior art single-gate or two-gate transistors are used for functions of gain control, modulation or mixing, they operate in the non-linearity region of their transconductance curve, namely in the vicinity of the pinch-off region for a single-gate transistor. This region corresponds to low transconductance values which change very quickly as a function of the level of the input signal, thus giving rise to either a small dynamic range or considerable distortion and very low conversion gain.
On the contrary, the transistor according to the invention does not use the non-linearity of transconductance. Through its two channels, it separates the "signal" and "control" paths and, if it is used as a mixer for example, it operates by multiplication.
The transistor according to the invention has a semiconductor body consisting of a substrate and different layers which need not be examined in detail since the invention concerns contact-making metallizations. A source metallization and a gate metallization are deposited on the upper surface of this semiconductor body and, at the locations made necessary by the internal structure of the transistor (depending on whether it has contact-making pads, or a mesa or a recess etc.). This source metallization and gate metallization conform to the prior art and form part of the main transistor. But the metallization of the drain of this transistor is divided into two parts. The two parts of the drain metallization have no contact with each other (unless it is through the external bias circuit) and the distance between them is enough for a second gate metallization to be inserted therein. This second gate is in a channel which is distinct from the channel of the main transistor. The two drain metalizations and the second gate metalization define a secondary field-effect transistor in the subjacent layers, the control of this secondary field-effect transistor makes it possible to couple the two drains variably.